This invention relates to audio speakers and more particularly to a speaker transducer system which is light-weight and able to produce high quality sound.
High-quality prior art speakers have generally employed transducer systems comprising a permanent magnet and an electrical coil. An audio signal, corresponding to sounds to be generated, would be directed to the coil. The coil then would create a time-varying magnetic field which would interact with the field of the permanent magnet to drive the speaker. This has limited such speakers to relatively fixed installations due to the weight of the magnet.
Somewhat recently the magnet weight problem has been addressed by replacing the magnet with a second electrical coil, as disclosed in Dinh U.S. Pat. No. 5,487,114. The Dinh prior art is illustrated in FIG. 1 hereof, as indicated by the reference numeral 30.
Referring now to FIG. 1, a speaker 42 is oscillated in correspondence with an audio signal 36 through forces of attraction and repulsion acting on a movable coil 34. Signal 36 is applied across input lines 38, 40. A stationary coil 32 is connected to lines 38,40 by lead lines 46, 44. Coil 32 generates magnetic flux having a field strength which generally follows the amplitude of signal 36. When signal 36 changes sign the flux generated by stationary coil 32 reverses direction.
Moveable coil 34 is connected to input line 38 by a bridge arrangement 48, comprising 4 diodes 48a-48d. Bridge 48 connects moveable coil 34 in parallel with stationary coil 32 but rectifies the current flowing through the moveable coil. This rectified current causes moveable coil 34 to generate magnetic flux which follows the amplitude of signal 36, but without reversal. Thus moveable coil 34 is alternately attracted to and repelled by stationary coil 32.
The magnetic field at coil 34 due to coil 32 is proportional to sin(.omega.t), where .omega. is the frequency of the audio signal 36, and the acceleration force acting on coil 34 is proportional to sin(.omega.t)ABS(sin(.omega.t)). Ideally the acceleration force should be proportional simply to sin(.omega.t), so the factor ABS(sin(.omega.t)) introduces a small distortion error. The bridge diodes cannot switch at zero volts. A practical threshold is in a region of about 0.5 to 0.7 volts. Since bridge 48 places two diodes in series, the moveable coil cannot respond to signal reversals below about 1 volt. This is fairly substantial in a typical 5 volt system.